The present invention relates to a screening device and a screening machine having a screening device for screening in particular biomass and similar bulk goods or the like. The screening machine and the screening device can be employed for a great variety of purposes. One possible field of application is the screening of biomass for thermal processing. The burning of biomass for generating heat or energy requires screening of the biomass provided so as to feed the desired useful grain fraction to the incinerator wherein for example sand or the like particles are first sifted out to reduce e.g. the ash content. It may furthermore be useful to sort out too large pieces so as to ensure a homogeneous, even incineration or prevent interferences in the conveyor devices of incinerators.
Similar conditions apply to other applications so as to render it useful to split the mass to be screened into two, three or more fractions one of which is processed further while waste materials such as sand or the like are for example immediately dumped. Any too large pieces can be chopped or shredded further and screened once again.
Screening biomass is for example carried out employing star screens where screen shafts aligned in parallel have screen stars attached on them which consist of elastic materials and comprise screen fingers extending radially outwardly from the shaft. Due to the elasticity of the screen fingers each finger can deflect resiliently as needed. The screen stars of a screen shaft each engage in clearances between the screen stars of an adjacent screen shaft. In operation the screen shafts rotate, thus conveying the material to be screened along the screen deck. Material to be screened gets between the screen fingers and is conveyed downwardly beneath the screen deck primarily by way of the screen stars rotating if the material received between the fingers is small enough.
These star screens having screen stars of elastic materials operate reliably. It is a drawback though that each of the screen star fingers requires a certain spatial extension to ensure the required stability. Therefore star screens of elastic materials cannot be used for screening fine and ultrafine biomass components.
Disk screens or disk separators have become known in the prior art in which multiple shafts are provided disposed in parallel in a rack and drivable non-rotatably on which multiple disks are arranged spaced apart and interdigitated. Such a disk screen has become known for example from EP 0861696 A1. In this known disk screen or disk separator the polygonal disks disposed on the shafts consist of a ferrous material. Parts of the disk body surfaces are provided with a hard-metal armor to prolong service life.
This known disk screen is thus basically also suitable for separating difficult-to-screen masses. Biomass tends to be a difficult-to-screen substance and moreover, being a cohesive screen feed, tends to cake and accumulate on the disks or the shaft hubs.
Metal screen disks allow selective screening of fine products. There is the drawback though that the screening of cohesive materials may involve increased caking of the screen feed on the disks and/or shaft hubs which may result in reduced screening output and possible blocking of the screen if more operating energy is required than operating energy is available.
Therefore it has become known in EP 0861696 A1 to provide every other tip of each disk with a laterally protruding scraper to clear caking from the clearance between the disks and the shaft hub surface during the rotary movement of the disks. This prior art is basically functional. There is the drawback, however, that the large quantity of 5 scrapers per disk involves high energy requirement for the rotational movement of the disk screens since the scrapers must continuously pass through the screened mass. Moreover caking may occur if fine, cohesive materials are screened.